This invention relates generally to the fields of lithography and semiconductor fabrication. More particularly, the invention relates to the use of certain fluorinated polymers useful in chemical amplification photoresist compositions, including ultraviolet, electron-beam, and x-ray photoresists.
There is a desire in the industry for higher circuit density in microelectronic devices made using lithographic techniques. One method of increasing the number of components per chip is to decrease the minimum feature size on the chip, which requires higher lithographic resolution. This has been accomplished over the past twenty years by reducing the wavelength of the imaging radiation from the visible (436 nm) down through the ultraviolet (365 nm) to the deep ultraviolet (DUV) at 248 nm. Development of commercial lithographic processes using ultra-deep ultraviolet radiation, particularly 193 nm, is now becoming of interest. See, for example, Allen et al. (1995), xe2x80x9cResolution and Etch Resistance of a Family of 193 nm Positive Resists,xe2x80x9d J. Photopolym. Sci. and Tech. 8(4):623-636, and Abe et al. (1995), xe2x80x9cStudy of ArF Resist Material in Terms of Transparency and Dry Etch Resistance,xe2x80x9d J Photopolym. Sci. and Tech. 8(4):637-642. The resists proposed for use with 193 nm imaging radiation do not appear suitable for use with 157 nm radiation due to their poor transparency at the 157 nm wavelength.
Certain attempts have been made to develop 157 nm resists, for example using heavily fluorinated materials such as polytetrafluoroethylene (e.g., Teflon AF(copyright); see Endert et al. (1999) Proc. SPIE-Int. Soc. Opt. Eng,3618:413-417) or hydridosilsesquioxanes (see U.S. Pat. No. 6,087,064 to Lin et al.). These materials do not, however, have the requisite reactivity or solubility characteristics. The challenge in developing chemically amplified resists for 157 nm lithography is in achieving suitable transparency in polymers that have acid-labile functionalities and developed with industry standard developers in either exposed or unexposed areas depending on whether the resist is positive or negative.
Polymers prepared from trifluoromethyl-substituted acrylates have been described. See, for example, Ito et al. (1981), xe2x80x9cMethyl Alpha-Trifluoromethylacrylate, an E-Beam and UV Resist,xe2x80x9d IBM Technical Disclosure Bulletin 24(4):991, Ito et al. (1982) Macromolecules 15:915-920, which describes preparation of poly(methyl xcex1-trifluoromethylacrylate) and poly(xcex1-trifluoromethylacrylonitrile) from their respective monomers, and Ito et al. (1987), xe2x80x9cAnionic Polymerization of xcex1-(Trifluoromethyl)Acrylate,xe2x80x9d in Recent Advances in Anionic Polymerization, T. E. Hogen-Esch and J. Smid, Eds. (Elsevier Science Publishing Co., Inc.), which describes an anionic polymerization method for preparing polymers of trifluoromethylacrylate. Willson et al., Polymer Engineering and Science 23(18):1000-1003, also discuss poly(methyl xcex1-trifluoromethylacrylate) and use thereof in a positive electron beam resist. However, none of these references discloses the utility of trifluoromethyl-substituted acrylate polymers in chemical amplification resists.
Accordingly, it is a primary object of the invention to address the above-described need in the art by providing a novel lithographic photoresist composition containing a fluorinated vinylic polymer.
It is another object of the invention to provide such a composition wherein the fluorinated vinylic polymer is substantially transparent to deep ultraviolet radiation, i.e., radiation having a wavelength less than 250 nm.
It is yet another object of the invention to provide such a composition wherein the fluorinated vinylic polymer is a fluorinated methacrylate, a fluorinated methacrylic acid, or a fluorinated methacrylonitrile.
It is still another object of the invention to provide a method for generating a resist image on a substrate using a photoresist composition as described herein.
It is a further object of the invention to provide a method for forming a patterned structure on a substrate by transferring the aforementioned resist image to the underlying substrate material, e.g., by etching.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention.
The inventors have now discovered that certain fluorinated polymers, particularly fluorinated acrylate polymers, exhibit surprisingly good transparency at 157 nm. The finding that an acrylate polymer can serve as a suitable 157 nm resist material in a chemically amplified resist is quite unexpected in view of the fact that the acrylate""s carbonyl functionality is highly absorbent at this wavelength. While incorporation of fluorine into polymers is known to lower absorbance at short wavelengths in at least some cases, it was anticipated that the relatively low fluorine content in polymers formed from methyl trifluoromethylacrylate (MTFMA), coupled with the fact that the trifluoromethyl group is insulated from the carbonyl moiety, would be insufficient to offset the very intense absorbance of the carbonyl group at 157 nm. Surprisingly, however, the incorporation of a trifluoromethyl group has turned out to substantially lower the absorbance of the carbonyl group at 157 nm. Therefore, polymers prepared from trifluoromethyl-substituted acrylates and derivatives thereof can be used to formulate chemical amplification resists useful in 157 nm lithography.
In one embodiment, then the present invention relates to a novel lithographic photoresist composition comprising a fluorinated vinylic polymer and a photosensitive acid generator (also referred to herein as a xe2x80x9cphotoacid generator,xe2x80x9d a xe2x80x9cPAG,xe2x80x9d or a xe2x80x9cradiation-sensitive acid generatorxe2x80x9d). The fluorinated vinylic polymer is preferably a fluorinated methacrylate, a fluorinated methacrylic acid, or a fluorinated methacrylonitrile. The fluorinated vinylic polymer preferably contains monomeric units having the structure (I) 
wherein R1 is fluoro, methyl or trifluoromethyl, preferably fluoro, and R2 is selected from the group consisting of 
wherein R3 is hydrogen, lower alkyl or fluorinated lower alkyl, or is selected so as to render R2 acid-cleavable. R4 and R5 are lower alkyl or are linked to form a five-or six-membered heterocyclic ring that may or may not contain an additional heteroatom, R6 and R7 are lower alkyl or are linked to form a five-or six-membered heterocyclic ring that may or may not contain an additional heteroatom and/or a carbonyl moiety, m is an integer in the range of 1 to 3 inclusive, n is zero or 1, and L is a linking group such as an alkylene (typically lower alkylene) chain or a phenylene ring. Preferably, R2 has the structure of formula (III), and R3 is selected so as to render R2 an acid-cleavable functionality, i.e., a molecular moiety that is cleavable with acid, particularly photogenerated acid. When R2 is xe2x80x94(CO)xe2x80x94OR3 (i.e., structure III), it will be appreciated that acid cleavage of the ester generates a free carboxylic acid group. The fluorinated polymer containing the aforementioned monomer unit may be a homopolymer or copolymer; if a copolymer, suitable comonomers will generally be additional polymerizable, ethylenically unsaturated comonomers, preferably vinyl comonomers such as acrylic acid and methacrylic acid derivatives, e.g., acrylates and methacrylates, styrene, substituted styrenes, norbornene, substituted norbornene, etc. In addition, the copolymer may contain two or more monomer units having the structure of formula (I), e.g., monomer units containing acid-cleavable pendant groups (such as xe2x80x94(CO)xe2x80x94OR3 groups wherein R3 renders the ester acid-cleavable), as well as other monomer units that provide for better dissolution, transparency, and/or other desirable properties of the resist composition (e.g., monomer units containing pendant xe2x80x94(CO)xe2x80x94Oxe2x80x94(lower alkyl) groups or xe2x80x94(CO)xe2x80x94Oxe2x80x94(fluorinated lower alkyl) groups, or the like). The polymer may serve as either the base-soluble component of an unexposed resist or as an acid-labile material (e.g., by virtue of containing acid-cleavable pendant groups such as acid-cleavable esters) that releases acid following irradiation by virtue of the photoacid generator in the resist composition.
The present invention also relates to the use of the resist composition in a lithography method. The process involves the steps of (a) coating a substrate (e.g., a ceramic, metal or semiconductor substrate) with a film comprising a radiation-sensitive acid generator and a fluorinated polymer as provided herein; (b) exposing the film selectively to a predetermined pattern of radiation to form a latent image therein; and (c) developing the image using a suitable developer composition. The radiation may be ultraviolet, electron beam or x-ray. Ultraviolet radiation is preferred, particularly deep ultraviolet radiation having a wavelength of less than about 250 nm, e.g., 157 nm, 193 nm, or 248 nm. The pattern from the resist structure may then be transferred to the underlying substrate. Typically, the transfer is achieved by reactive ion etching or some other etching technique. Thus, the compositions of the invention and resulting resist structures can be used to create patterned material layer structures such as metal wiring lines, holes for contacts or vias, insulation sections (e.g., damascene trenches or shallow trench isolation), trenches for capacitor structures, etc. as might be used in the design of integrated circuit devices.